Partially hydrolyzed aluminum alkyl compounds known as aluminoxanes (a.k.a. alumoxanes) are effective in activating metallocenes for polymerization of olefins. Activating effects of water in such systems were initially noted by Reichert, et al. (1973) and Breslow, et al. (1975), and extended to trimethylaluminum-based systems by Sinn, Kaminsky, et al. (1976). Subsequent research by Sinn and Kaminsky demonstrated that this activation was due to formation of methylaluminoxane from partial hydrolysis of trimethylaluminum present in the system. Methylaluminoxane (a.k.a. methylalumoxane) has become the aluminum co-catalyst of choice in the industry.
Subsequent to the above original discoveries in this field, considerable worldwide effort has been devoted to improving the effectiveness of catalyst systems based on use of aluminoxanes or modified aluminoxanes for polymerization of olefins and related unsaturated monomers.
Representative of many patents in the field of aluminoxane usage in forming olefin polymerization catalyst systems with suitable metal compounds is U.S. Pat. No. 5,324,800 to Welborn et al. which claims an original filing date in 1983. This patent describes olefin polymerization catalysts made from metallocenes of a metal of Groups 4b, 5b, or 6b, and a cyclic or linear C.sub.1 -C.sub.5 alkylaluminoxane. The cyclic and the linear aluminoxanes are depicted, respectively, by the formulas (R--Al--O).sub.n and R(R--Al--O).sub.n AlR.sub.2 where n is from 1 to about 20, and R is most preferably methyl. The aluminoxanes are made by controlled hydrolysis of the corresponding aluminum trialkyl.
Another relatively early patent in the field, U.S. Pat. No. 4,752,597 to Turner based on a filing date of 1985, describes olefin polymerization catalysts comprising the reaction products of a metallocene complex of group IVB, VB, VIB, and VIII of the periodic table and an excess of aluminoxane. These catalysts are formed by pre-reacting a metallocene and an aluminoxane in mole ratios greater than 12:1, such as about 12:1 to about 100:1, to produce a solid product which precipitates from solution. Despite assertions of suitable catalytic activity, in reality the activity of these materials is so low as to be of no practical importance whatsoever.
In U.S. Pat. Nos. 4,960,878 and 5,041,584 to Crapo et al. modified methylaluminoxane is formed in several ways. One involves reacting a tetraalkyldialuminoxane, R.sub.2 Al--O--AlR.sub.2, containing C.sub.2 or higher alkyl groups with trimethylaluminum (TMA) at -10 to 150.degree. C. Another involves reacting TMA with a polyalkylaluminoxane (--Al(R)--O--).sub.n where R is C.sub.2 alkyl or higher and n is greater than 1, e.g., up to 50. Temperatures suggested for this reaction are -20 to 50.degree. C. A third way involves conducting the latter reaction and then reacting the resultant product, which is indicated to be a complex between trimethylaluminum and the polyalkylaluminoxane, with water. The patent states that the water-to-aluminum ratios used to make the polyalkylaluminoxane reagent have an effect on the activity of the final methylaluminoxane. On the basis of ethylene polymerizations using zirconocene dichloride catalyst and a complex of trimethylaluminum with polyisobutylaluminoxane subsequently reacted with water (MMAO) as co-catalyst, it is indicated in the patent that the highest polymerization activities were achieved with MMAO co-catalyst prepared at H.sub.2 O/Al ratios of about 0.6 to about 1.0 and Al/Zr ratios in the range of 10,000/1 to 400,000/1.
Various references are available indicating that isobutylaluminoxanes themselves are relatively ineffective on their own as co-catalysts. For example, several other reactions of alkylaluminum compounds with water are disclosed in U.S. Pat. Nos. 4,960,878 and 5,041,584. Thus in Example 1 of these patents, DIBAL-O (tetraisobutyldialuminoxane), a commercial product, was prepared by reaction of water with triisobutylaluminum (TIBA) in heptane using a water/TIBA ratio of about 0.5, followed by solvent stripping at 58-65.degree. C. under vacuum. In Examples 3-6 of the patents isobutylaluminoxane (IBAO) was prepared by controlled addition of water to a 25% solution of TIBA in toluene in the temperature range of 0-12.degree. C., followed by heating to 70-80.degree. C. to ensure complete reaction and remove dissolved isobutane. H.sub.2 O/Al ratios used were 0.98, 1.21, 1.14, and 0.88. IBAO was again made in a similar manner in Example 52 of the patents. Here the H.sub.2 O/Al ratio was 0.70, and the product was heated at 75.degree. C. And in Example 70 tri-n-butylaluminum (TNBA) in toluene was treated at 0-10.degree. C. with water followed by heating to 85.degree. C. Ethylene polymerizations using zirconocene dichloride catalyst and various products from the foregoing Examples were conducted. Specific activities (.times.10.sup.3 gPE/(gZr.atm C.sub.2 H.sub.4.hr)) of the catalysts made with DIBAL-O from Ex. 1, IBAO from Ex. 3, and IBAO from Ex. 6 were, respectively, 4.1, 4.2, and 7.7, as compared to 1000 for the catalyst made using conventional MAO as the co-catalyst. The patents acknowledge that tetraisobutyldialuminoxane (DIBAL-O) showed "poor polymerization activity", and from the foregoing test results the same can be said to apply to IBAO.
WO 96/02580 to Dall'occo, et al. describes olefin polymerization catalysts made by contacting a metallocene of Ti, Zr, or Hf, an organoaluminum compound having at least one specified hydrocarbon substituent on the .beta.-carbon atom of an aliphatic group bonded to an aluminum atom, and water. Various ways of bringing these components together are suggested. Polymerizations described were carried out using Al/Zr mole ratios ranging from 500 up to as high as 5000.
EP 0 277 004 to Turner, published in 1988, describes the successful preparation and use as catalysts composed of an ionic pair derived from certain metallocenes of Group 4, most preferably bis(cyclopentadienyl)zirconium dimethyl or bis(cyclopentadienyl)hafnium dimethyl, reacted with certain trisubstituted ammonium salts of a substituted or unsubstituted aromatic boron compound, most preferably N,N-dimethylanilinium tetra(pentafluorophenyl)boron. While EP 0 277 004 mentions that compounds containing an element of Groups V-B, VI-B, VII-B, VIII, I-B, II-B, III-A, IV-A, and V-A may be used in forming the catalysts, no specific compounds other than boron compounds are identified. In fact, EP 0 277 004 appears to acknowledge inability to identify specific compounds other than boron compounds by stating: "Similar lists of suitable compounds containing other metals and metalloids which are useful as second components could be made, but such lists are not deemed necessary to a complete disclosure." See in this connection Hlatky, Turner and Eckman, J. Am. Chem. Soc., 1989, 111, 2728-2729, and Hlatky and Upton, Macromolecules, 1996, 29, 8019-8020.
U.S. Pat. No. 5,153,157 to Hlatky and Turner states that its process "is practiced with that class of ionic catalysts referred to, disclosed, and described in European Patent Applications 277,003 and 277,004." The process of U.S. Pat. No. 5,153,157 involves forming an ionic catalyst system from two components. The first is a bis(cyclopentadienyl) derivative of a Group IV-B metal compound containing at least one ligand which will combine with the second component or portion thereof such as a cation portion thereof. The second component is referred to as an ion exchange compound comprising (1) a cation which will irreversibly react with a ligand of the Group IV-B metal compound and (2) a noncoordinating anion which is bulky, labile, and stable. The second component, also termed an activator component, comprises compounds of Groups V-B, VI-B, VII-B, VIII, I-B, II-B, III-A, IV-A, and V-A identified by a general formula. Besides referring to the boron compounds of EP 277,004, supra, such as tri(n-butylammonium)tetra(pentafluorophenyl)boron and N,N-dimethylanilinium tetra(pentafluorophenyl)boron as suitable activators, the U.S. '157 patent teaches use of boron compounds having a plurality of boron atoms, and also trialkyl aluminum compounds, triaryl aluminum compounds, dialkylaluminum alkoxides, diarylaluminum alkoxides, and analogous compounds of boron. Of the organoaluminum activators triethylaluminum and trimethylaluminum are specified as most preferred. The Examples show use of a catalyst system formed from (1) a solution of bis(cyclopentadienyl)zirconium dinethyl or bis(cyclopentadienyl)hafnium dimethyl and N,N-dimethylanilinium tetra(pentafluorophenyl)boron together with (2) triethylborane, triethylaluminum, tri-sec-butylborane, trimethylaluminum, and diethylaluminum ethoxide. In some cases the catalyst formed from the metallocene and the N,N-dimethylanilinium tetra(pentafluorophenyl)boron without use of a compound of (2) gave no polymer at all under the polymerization conditions used.
U.S. Pat. No. 5,198,401 to Turner, Hlatky, and Eckman refers, in part, to forming catalyst compositions derived from certain metallocenes of Group 4, such as bis(cyclopentadienyl)zirconium dinethyl or bis(cyclopentadienyl)hafnium dimethyl, reacted with certain trisubstituted ammonium salts of a substituted or unsubstituted aromatic boron compound, such as N,N-dimethylanilinium tetra(pentafluorophenyl)boron or tributylammonium tetra(pentafluorophenyl)boron as in EP 0 277 004. However here the anion is described as being any stable and bulky anionic complex having the following molecular attributes: 1) the anion should have a molecular diameter about or greater than 4 angstroms; 2) the anion should form stable salts with reducible Lewis acids and protonated Lewis bases; 3) the negative charge on the anion should be delocalized over the framework of the anion or be localized within the core of the anion; 4) the anion should be a relatively poor nucleophile; and 5) the anion should not be a powerful reducing or oxidizing agent. Anions of this type are identified as polynuclear boranes, carboranes, metallacarboranes, polyoxoanions and anionic coordination complexes. Elsewhere in the patent it is indicated that any metal or metalloid capable of forming a coordination complex which is resistant to degradation by water (or other Br.o slashed.nsted or Lewis acids) may be used or contained in the second activator compound [the first activator compound appears not to be disclosed]. Suitable metals of the second activator compound are stated to include, but not be limited to, aluminum, gold, platinum and the like. No such compound is identified. Again after listing boron compounds the statement is made that "Similar lists of suitable compounds containing other metals and metalloids which are useful as second components could be made, but such lists are not deemed necessary to a complete disclosure." In this connection, again note Hlatky, Turner and Eckman, J. Am. Chem. Soc., 1989, 111, 2728-2729, and Hlatky and Upton, Macromolecules, 1996, 29, 8019-8020.
Despite the above and many other efforts involving aluminum co-catalysts, the fact remains that in order to achieve suitable catalysis on a commercial basis, relatively high aluminum to transition metal ratios must be employed. Typically for optimal activity an aluminum to metallocene ratio of greater than about 1000:1 is required for effective homogeneous olefin polymerization. According to Brintzinger, et al., Angew. Chem. Int. Ed. Engl., 1995, 34 1143-1170:
"Catalytic activities are found to decline dramatically for MAO concentrations below Al:Zr ratio roughly 200-300:1. Even at Al:Zr ratios greater than 1000:1 steady state activities increase with rising MAO concentrations approximately as the cube root of the MAO concentration".
This requirement of high aluminum loading is mainly caused by a metallocene activation mechanism in which generation of catalytically active species is equilibrium driven. In this role MAO acts as a Lewis acid to remove by group transfer a leaving group X.sup..crclbar. from the transition metal. This forms a weakly-coordinating anion, MAO-X.sup..crclbar., in the corresponding transition metal cation. That is, in such systems the following equilibrium exists: ##EQU1## The Lewis acid sites in MAO abstract a negatively charged leaving group such as a methide group from the metallocene to form the catalytically active ion pair. The activation process is reversible and K.sub.eq is typically small. Thus the ion pair can return to its neutral precursors which are catalytically inactive. To overcome this effect, a large excess of MAO is required to drive the equilibrium to the right.
The high aluminum loadings required for effective catalysis in such systems result in the presence of significant levels of aluminum-containing residues ("ash") in the polymer. This can impair the clarity of finished polymers formed from such catalyst systems.
A further disadvantage of MAO is its limited solubility in paraffinic hydrocarbon solvents. Polymer manufacturers would find it of considerable advantage to have in hand aluminoxane and metallocene-based materials having high paraffin solubility.
Still another disadvantage of MAO has been its relatively high cost. For example, in an article entitled "Economics is Key to Adoption of Metallocene Catalysts" in the Sep. 11, 1995 issue of Chemical & Engineering News, Brockmeier of Argonne National Laboratory concluded that "a reduction in costs or amount of MAO has the potential for greatly reducing the costs to employ metallocene catalysts".
Thus it would be of inestimable value to the art if a way could be found of providing catalyst components based on use of aluminoxanes that are effective co-catalysts for use with transition metal compounds at much lower aluminum:metal ratios than have been effective heretofore. In addition, the art would be greatly advanced if this could be accomplished with aluminoxane compositions that are less expensive than MAO, that have high solubility in paraffinic solvents and that produce lower ash residues in the polymers.
This invention is deemed to have fulfilled most, if not all, of the foregoing desirable objectives.